1. Technical Field
The present invention relates to an apparatus and a method for controlling the temperature of an element of an exhaust gas sensor provided in an exhaust system of an internal combustion engine.
2. Description of the Related Art
A catalyst converter is provided in an exhaust system of an internal combustion engine of a vehicle. When the air-fuel ratio of air-fuel mixture introduced into the engine is lean, the catalyst converter oxidizes HC and CO with excessive oxygen included in the exhaust gas. When the air-fuel ratio is rich, the catalyst converter reduces Nox with HC and CO. When air-fuel ratio is in the stoichiometric air-fuel ratio region, HC, CO and Nox are simultaneously and effectively purified.
An exhaust gas sensor is provided downstream of the catalyst converter. The exhaust gas sensor detects the concentration of oxygen included in the gas discharged into the exhaust system. The detection value of the exhaust gas sensor is used for various control of the internal combustion engine. If response of the exhaust gas sensor deteriorates, the internal combustion engine may not be appropriately controlled, which may cause deterioration in the operating state of the engine.
The output of the exhaust gas sensor varies according to the temperature of an element provided in the exhaust gas sensor. The element is typically zirconium (ZrO2) or titania (TiO2). The air-fuel ratio is controlled by converging the output of the exhaust gas sensor to a desired value. If the element temperature changes, the desired value established for the air-fuel ratio control also changes. In order to implement stable air-fuel control, the element temperature needs to be controlled.
The element is heated by a heater, which is typically provided in the exhaust gas sensor. The element temperature is adjusted by controlling a duty ratio of energization of the heater. For example, the energization duty ratio is determined based on an elapsed time after the engine start.
Japanese Patent Application Unexamined Publication No. 2000-304721 describes another scheme in which an energization duty ratio of the heater is determined by referring to a table or map that is pre-established in accordance with the temperature of an air-fuel ratio sensor. According to the scheme, the energization duty ratio is further corrected in accordance with the operating state (e.g., fuel cut state, idling state) of the engine.
When the element temperature is controlled to a temperature that is different from a temperature used for the normal operating state of the engine, the accuracy of the control for the element temperature needs to be maintained at high level. If the element temperature is not controlled to an appropriate temperature, overshoot or undershoot of the element temperature may occur, which may damage the heater and inactivate the element.
In the above conventional scheme in which the energization duty ratio of the element is determined by referring to the preset table or map, it is difficult to precisely control the element temperature to a desired temperature.
Therefore, there is a need for an apparatus and a method that stably and precisely controls the element temperature of an exhaust gas sensor.